The present invention relates to a method for stabilizing organic polymers against copper-catalyzed oxidative decomposition through use of N,N'-bis-salicyloyl hydrazine as a metal deactivator.
Many organic materials, particularly polymers such as polyolefins, polyoxymethylenes, polyamides and unsaturated polyester resins which are being used today in electrical engineering for insulating purposes, are subjected to accelerated thermo-oxidative aging in the presence of copper. This aging causes the electrical and mechanical use properties of such polymers to deteriorate considerably. Particularly aggravating is the harmful effect of copper at elevated temperatures, since the aging rate of the polymers increases steeply with increasing temperatures.
Cross-linked polyolefins, which currently are being employed more and more as insulating material for cables or wires, are particularly subject to heavily accelerated aging in the presence of copper and must therefore be protected effectively against the oxidation-accelerating influence of the copper. According to experience, this protection can be accomplished by placing a foil between the copper conductor and the insulation as a separting, layer, or by using tinned conductors, thereby preventing direct contact of the copper and the insulation. Such measures are expensive, however, and require considerable effort in production. This, if a foil is used as a separating layer, only low production rates are possible, particularly for small conductor cross sections.
Another approach to achieving the desired requirements with respect to quality and temperature resistance of polymer materials in contact with copper is the use of so-called "copper deactivators" which inhibit the oxidation-accelerating effect of copper even at elevated temperatures. The stabilization of polymer materials by copper deactivators is much simpler, from a production point of view, than the use of foil separating layers and is, therefore, a cost-effective measure.
It is known from U.S. Pat. No. 3,849,492 to use copper deactivators of the N,N'-bis-salicyloyl-hydrazine type. Multiply alkyl- or alkoxy-substituted derivatives of the base compound are taught by this patent to have particularly proven themselves for the stabilization of polyolefins against the harmful effect of copper and other transition metals.
It is known from British Patent Specification No. 1,593,902 that for the permanent stabilization of polymers which are in contact with copper, a combination of N,N'-bis-salicyloyl hydrazine as a metal deactivator and oligomeric 2,2,4-trimethyl-1,2-dihydroquinoline as an oxidation inhibitor is commercially advantageous. This stabilizer combination has been found to be particularly effective for cross-linked polyolefins. The high effectiveness of N,N'-bis-salicyloyl hydrazine as a metal deactivator was determined on the basis of extensive tests on model conductors and through aging tests on commercial products.
A metal deactivator of the type mentioned (as well as other additives) can be worked into a polymer material, such as into a polyolefin for cable or wire insulation, on a commercial scale by customary mixing processes. In cable and wire technology, so-called "concentrates" consisting of a metal deactivator or other additives and polymer material are preferably prepared first, and these concentrates are then processed further by admixture into further polymer material to form insulation mixtures with the desired concentrations of metal deactivator and the other additives. In the preparation of the concentrates with the metal deactivator N,N'-bis-salicyloyl hydrazine on a commercial production scale, it has been found that eye irritation or eye damage occurs in persons concerned therewith.
To determine the cause of the eye irritation or eye damage, tests with rabbits were performed. Since it must be assumed that the metal deactivator can, during the processing, get into the eye in solid form, for example, as dust, the solid substance repeatedly with placed at relatively short time intervals, i.e., daily, in the eye of the test animals, and the changes in the eye were observed over an extended period of time. As a result of these tests it was determined that the eye irritations and eye damage were caused by the N,N'-bis-salicyloyl hydrazine or by a small amount of impurities contained therein due to its manner of preparation.
According to South-African Patent 80/5131 and German Offenlegungsschrift 30 33 383, greatly reduced eye irritations or eye damage occur if an N,N'-bis-salicyloyl hydrazine is used which is obtained by reaction of alkyl salicylate with either hydrazine or salicylic acid hydrazide. However, the products obtained in this manner are not completely safe physilogically.